1. Field of the Invention
The present invention relates generally to radars and, more particularly, to systems and methods for monitoring river flow parameters using a VHF/UHF radar station.
2. Description of Related Art
The monitoring of river flow, and in particular the volume discharge past a point as a function of time, is a well established technology dating back over a century. There are thousands of stream gauging systems in use in the U.S. at present. Many citizens depend on the free-access availability of river discharge for decisions in agriculture, flood control/monitoring, boat operations, etc. Unfortunately, conventional methods all have the disadvantage of using sensors placed in the water, and hence their lifetimes are limited and maintenance costs are high. As such, the need for “non-contact” replacements has been articulated by many, including the U.S. Geological Survey, state agencies, etc. Obtaining a velocity profile across the river is now considered an acceptable input data set to the process of estimating the total river discharge.
High frequency (HF) radars have found extensive application for mapping currents on the ocean surface. Part of the reason for this is that their long wavelength signals, when scattered from the dominant sea-surface waves, lead to a very simple, robust physical and phenomenological interpretation that is not possible with much more widely utilized microwave radars. Currents or surface flow patterns give rise to Doppler shifts from Bragg-scattering waves, i.e., those half the radar wavelength moving toward and away from the radar. Currents transporting the Bragg waves impart an additional Doppler shift from that due to the known wave-induced velocity, and the former may be extracted based on knowledge of the latter. Two or more radars on the coast viewing the same point on the sea allow a total horizontal velocity vector to be constructed at each map point from the radials. U.S. Pat. Nos. 4,172,255 and 5,361,072, describe the technology of HF radar coastal current mapping radars, while U.S. Pat. No. 5,990,834 describes how bearing for these current-mapping radars are determined using highly compact antennas. HF radars viewing a river surface, however, are not suitable because the long Bragg waves corresponding to HF wavelengths are not present on smaller-scale rivers and channels.
Microwave radars whose wavelengths span a couple centimeters have been tried for river velocity profiling. Doppler versions that also do precise range measurement have the disadvantage that they are expensive and therefore less attractive for widespread use. Another disadvantage is the complex scattering mechanism due to their very short wavelengths; this leads to inaccuracies in water velocity extraction, because a simple Bragg-wave dispersion relation that works so well at HF on the sea and ultra high frequency (UHF) on rivers is not applicable; less accurate empirical rules must be established. On the other hand, forming narrow beams with their parabolic dish antennas is a well understood concept. An example of use of microwave radars for river monitoring may be found in “Measurement of River Surface Currents with Coherent Microwave Systems,” Plant et al., IEEE Trans. Geoscience & Remote Sensing, Vol. 43, No. 6, pp. 1242-1257, 2005.
To achieve the same narrow beamwidth at UHF that microwave radars possess would demand an antenna tens of meters in size. This presents a significant obstacle to acceptance, both from structural size and cost standpoints. HF sea-current mapping radars with compact antennas in common use nowadays have gotten around the large size limitation by trading high directive gain for the sake of a broad field of view (up to 360°). U.S. Pat. No. 5,361,072 describes a direction-finding radar system comprised of compact, co-located with crossed-loop and monopole antennas.
An example of a UHF river-velocity monitoring radar is described in “UHF Surface Current Radar Hardware System Design,” Ma et al., IEEE Microwave and Wireless Components Letters, Vol. 15, No. 12, pp. 904-906, 2005. This system operates at 300 MHz and uses Yagi antennas on a riverbank. However, this Chinese system has two major limitations. First, Yagi antennas have quite broad beamwidth. When used by themselves in a conventional arrangement, they produce velocity profiles or maps with seriously degraded bearing resolution, leading to biases. Second, the system uses a CW (continuous-wave or non-pulsed signal format), which stresses the dynamic range of the receiver. To handle weak signals, the Ma et al. discuss the need for separate antennas for transmit and receive, with an interference-reducing fence between them. This constitutes a severe handicap to robust operation, because their antenna arrangement is no longer compact nor is it a low-cost system.
Thus, in order to realize the many advantages a UHF river-monitoring radar offers, the inventors hereof have recognized a number of obstacles to be overcome. The present invention solves these and other problems by providing a cost-effective VHF/UHF approach for real-time river flow and discharge monitoring.